linux-kernel - Re: [PATCH 48/74] x86, lto: Use inline assembler instead of global register variable to get sp

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Date:	Sun, 19 Aug 2012 09:37:27 +0100
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<andi@...stfloor.org>, <ak@...ux.intel.com>,
	<linux-kernel@...r.kernel.org>
Cc:	<x86@...nel.org>, <akpm@...ux-foundation.org>, <mmarek@...e.cz>,
	<linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH 48/74] x86, lto: Use inline assembler instead of
 global register variable to get sp

>>> Andi Kleen <andi@...stfloor.org> 08/19/12 4:59 AM >>>
>I verified this generates the same binary (on 64bit) as the original
>register variable.

This isn't very surprising given that the modified code is inside a
CONFIG_X86_32 conditional (as ought to be obvious from the code using
%%esp). Given that it's being used as operand to a binary &, the resulting
code - if the compiler handles this only half way sensibly - can hardly be
expected to be identical.

>-register unsigned long current_stack_pointer asm("esp") __used;
>+#define current_stack_pointer ({         \
>+    unsigned long sp;            \
>+    asm("mov %%esp,%0" : "=r" (sp));    \
>+    sp;                    \
>+})

It would get closer to the original if you used "=g" (I noticed in a few
earlier patches already that you like to use "=r" in places where a register
is not strictly required, thus reducing the flexibility the compiler has).

Also, given that this is more a workaround for a compiler deficiency,
shouldn't this be conditional upon use of LTO?

Jan

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